Poly(esteramides) are known to be useful bioabsorbable polymeric materials derived from reacting diamidediols with dicarboxylic acids, derivatives thereof or bischloroformates. Such polymers and some of their uses are described in U.S. Pat. Nos. 4,343,931; 4,529,792; 4,534,349; 4,669,474; 4,719,917; 4,883,618; and 5,013,315 (all Barrows et al.).
An increasing number of surgically implantable devices that only function for a relatively short period of time in vivo are being designed from synthetic polymers that are eliminated from the body by hydrolytic degradation and subsequent metabolism after serving their intended purpose. The molecular weight of such "bioabsorbable" polymers is an important parameter in determining whether or not a particular lot of polymer will perform properly for an adequate length of time in any specific application. Thus, if significant loss of molecular weight occurs during processing of the polymer to fabricate a device, such as by melt extrusion, then the device may fail prematurely in vivo as hydrolysis shifts the molecular weight distribution below a minimum range required for the device to function properly. Some causes which may lead to reduction in molecular weight and premature device failure include degradation due to melt processing conditions (e.g., the high temperature and shear conditions encountered during extrusion processing), due to moisture in the polymer, and due to synthesis residues (e.g., unreacted precursor materials and reaction by-products) in the polymer, e.g., acid chlorides.
Loss of molecular weight upon extrusion is commonly encountered with polyesters even when moisture in the resin has been reduced to the lowest possible level. For example, poly-L-lactide, a well known bioabsorbable polyester has not yet been melt extruded into fiber with molecular weight much higher than 100,000 since the extrusion process typically degrades higher molecular weight fractions to this relatively constant maximum value.
Bioabsorbable poly(esteramides) described in U.S. Pat. Nos. 4,343,931 and 4,529,792 offer an advantage over poly-L-lactide and related polyesters in that a lower molecular weight is adequate to achieve comparable fiber strength due to the intermolecular hydrogen bonding provided by the amide linkages. Fibers made of such polymers exhibit comparable strength to those made of poly-L-lactide while providing lower modulus (and thus greater flexibility) and greater toughness and durability. Compared to poly-L-lactide such fibers are also more rapidly bioabsorbed. A disadvantage with poly(esteramides), however, results from the unreliable method of synthesis in which intermediate molecular weight material must be heated as a solid to advance the molecular weight to an acceptable value. If continued too long, this treatment yields excessively crosslinked or "gelled" material which is unsuitable for extrusion. On the other hand, if the process is not continued long enough, the polymer lacks storage and thermal stability due to the presence of unreacted acid chloride functionality.